No-clean is a process, not a product

No-clean fluxes can save a lot of processing time, but residues that can be trapped under high density components are raising reliability concerns.

As boards get smaller and densities skyrocket, poor processing and handling can cause serious problems. When low profile packages are soldered onto boards, the effects of the flux residue on the long-term reliability of the device need to be properly understood.

Over the past few years, contaminants that remain after no-clean manufacturing processes have become a key area of concern. Many companies want to learn more about the electrical reliability issues caused by the residues that are trapped under components or RF shields used in cell phones and other communication products. That’s prompted a lot of interest in surface insulation resistance (SIR) testing, which helps determine whether these residues can impact reliability.

“No-clean processes leave residues that remain for the life of the product,” said Eric Bastow, senior technical support engineer at Indium Corp. “The industry needs to understand that residues underneath a component body or RD shield may not have the same SIR characteristics as exposed residues on the board surface.”

Bastow will be sharing information on SIR tests, as well as overall concerns about no-clean processes, in a Webinar on Wednesday, August 10 between 10:00 and11:00 am central time. His presentation, Effects of Flux Residues on Reliability, will include updates of his two IPC APEX EXPO sessions drew standing room only crowds.

He stresses that avoiding reliability problems requires taking a large view of the soldering process. Simply using materials that say no cleaning is required will not mean that the resulting residue is no-clean. It is heavily dependent on the process,” Bastow said.

Bastow builds his sessions around two key aspects of no-clean processes. The first is to explain SIR tests and the factors that impact them. This section is designed to help companies understand how they should prepare test boards. Those tests focus on the fluxes and other materials used in manufacturing, as well as general cleanliness.

“The tests are performed in a warm, humid chamber, representing the worst case scenario for these residues,” Bastow said. “For example, sweat has salt that remains after the human moisture dries. When the board is in a moist environment, the salt may facilitate current leakage or corrosion.”

The larger portion of the Webinar will focus on real world scenarios. One of the most common problems occurs when no-clean flux materials are trapped under the part or beneath an RF shield. Knowing when that can be an issue, and when it’s a harmless annoyance, is critical for manufacturing a reliable product. Understanding the conductive and corrosive nature of residues plays an important role in determining whether the residue should be removed or allowed to remain on the board post-reflow.

These issues are especially interesting to electronics manufacturing services companies and OEMs, who may want to verify a flux manufacturer’s SIR test results by performing their own testing. “These tests are increasingly popular as products like cell phones have several RF shields and more products have ball grid arrays and bottom termination components that have very low profiles,” Bastow said. “When they see flux trapped under these components, they want to be sure they’ve been wise in the products they choose. I’ll be discussing some of the possible outcomes in those scenarios.”